As used herein, the term EMI should be considered to refer generally to both electromagnetic interference and radio frequency interference (RFI) emissions, and the term “electromagnetic” should be considered to refer generally to electromagnetic and radio frequency.
During normal operation, electronic equipment typically generates undesirable electromagnetic energy that can interfere with the operation of proximately located electronic equipment due to EMI transmission by radiation and conduction. The electromagnetic energy can be of a wide range of wavelengths and frequencies. To minimize the problems associated with EMI, sources of undesirable electromagnetic energy may be shielded and electrically grounded. Alternatively, or additionally, susceptors of EMI may be similarly shielded and electrically grounded. Shielding is designed to prevent both ingress and egress of electromagnetic energy relative to a housing or other enclosure in which the electronic equipment is disposed. Since such enclosures often include gaps or seams between adjacent access panels and around doors, effective shielding is difficult to attain, because the gaps in the enclosure permit transference of EMI therethrough. Further, in the case of electrically conductive metal enclosures, these gaps can inhibit the beneficial Faraday Cage Effect by forming discontinuities in the conductivity of the enclosure which compromise the efficiency of the ground conduction path through the enclosure. Moreover, by presenting an electrical conductivity level at the gaps that is significantly different from that of the enclosure generally, the gaps can act as slot antennae, resulting in the enclosure itself becoming a secondary source of EMI.
Shields are generally constructed to reduce EMI at a particular wavelength, or range of wavelengths. EMI shields are typically constructed of a highly-conductive material operating to reflect the radiation component of the EMI and to drain to electrical ground the conducted component of the EMI. For example, EMI shields are typically constructed of a metal, such as copper, aluminum, gold, tin, steel, and stainless steel, sheet metal and nickel. EMI shields may also be constructed of combinations of different metals, such as nickel-coated copper, and combinations of a conductive material with an electrical insulator, such as metal-plated plastic. In the abstract, an ideal EMI shield would consist of a completely enclosed housing constructed of an infinitely-conductive material without any apertures, seams, gaps, or vents. Practical applications, however, result in an enclosure constructed of a finitely-conducting material and having some apertures. Generally, reducing the largest dimension (not merely the total area) of any aperture, as well as reducing the total number of apertures, tends to increase the EMI protection or shielding effectiveness of the enclosure. Apertures may be intentional, such as those accommodating air flow for cooling, or unintentional, such as those incident to a method of construction (e.g., seams). Special methods of manufacture may be employed to improve shielding effectiveness by welding or soldering seams, or by milling a cavity. The shielding effectiveness of an EMI enclosure having an aperture is a function of the wavelength of the EMI. Generally, the shielding effectiveness is improved when the largest dimension of the aperture is small compared to the wavelength (i.e., less than one-half the wavelength). As the frequencies of operation increase, however, the associated wavelengths of induced EMI decrease, leading to a reduction in shielding effectiveness for any non-ideal EMI enclosure.
EMI shielded enclosures are typically constructed of conductive materials that induce resonances of the electromagnetic energy within the cavity. For example, reflections of the electromagnetic field at the boundaries of the cavity can create standing waves within the cavity under certain conditions. Such resonances tend to increase the peak amplitudes of the electromagnetic energy through additive effects of the multiple reflections. These resonance effects, by increasing the peak energy levels within the enclosure, can reduce the apparent shielding effectiveness at the resonant frequencies because the same enclosure is shielding a larger source of EMI—the resonant peak electromagnetic energy.
EMI protection is particularly important in small, densely packaged, sensitive electronic applications operating at high frequencies. In one application, a communications transceiver, such as a Gigabit Interface Converter (GBIC), converts electrical currents into optical signals suitable for transmission over a fiber-optic cable and optical signals into electrical currents. GBICS are typically employed in fiber-optic telecommunications and networking systems as an interface for high-speed networking. As the name suggests, the data rates of transmission are greater than one gigabit-per-second (Gbps). In some applications GBIC modules are installed within an EMI enclosure. One particular form factor for an EMI cage 50, or housing, shown in FIGS. 1A and 1B is described in a Multi-source Agreement (MSA) prepared by several cooperating members within the related industry. As shown in FIG. 1, one end 55 of the housing 50 is opened to accommodate the insertion and extraction of a GBIC transceiver (i.e., a transceiver having a form factor compliant with the Small-Form-Factor-Pluggable specifications described in the “Cooperation Agreement for Small Form-Factor Pluggable Transceivers,” dated Sep. 14, 2000, the contents of which are herein incorporated by reference in their entirety). The MSA-recommended EMI cage 50 offers a design level of shielding effectiveness for GBIC operations at 1 Gbps; however, as operating frequencies increase, the shielding effectiveness of the recommended EMI cage, without modification, will be inadequate. For example, emerging applications using the optical carrier protocols described in the Synchronous Optical Network (SONET) standards can operate above 1 Gbps (e.g., the OC-48 protocol supporting data rates of up to 2.5 Gbps and OC-192 protocol supporting data rates of up to 10 Gbps).
There exist certain methods for providing EMI shielding to electronic components. For example, U.S. Pat. No. 5,639,989 issued to Higgins, III, the disclosure of which is herein incorporated by reference in its entirety. Higgins discloses the use of a housing wherein all interior surfaces are conformally coated with a first EMI material consisting of a polymer containing filler particles. The method disclosed in Higgins applies the first EMI material as a conformal coating. The disclosed method also indicates that selection of different materials for filler particles results in the attenuation of electromagnetic energy within specified frequency ranges.